1. Field of the Invention
The present invention relates to an imaging lens that forms an optical image of a subject on an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), a camera module that converts the optical image formed by the imaging lens into an imaging signal, and an imaging apparatus such as a cellular phone and a portable information terminal (PDA: Personal Digital Assistance) equipped with a camera or a digital still camera that performs photographing with the imaging lens mounted therein.
2. Description of Related Art
In recent years, as personal computers have become popular in homes, digital still cameras capable of transferring image information about scenes, persons, and the like photographed into the personal computers have spread rapidly. In addition, the incorporation of a camera module for inputting images into a cellular phone has been increasing. In these imaging apparatuses, there are used imaging devices such as a CCD and a CMOS. In such imaging apparatus, recently, because the imaging device has been downsized, it has been also required to downsize the whole of the imaging apparatus and an imaging lens mounted therein. Also, since the number of pixels included in the imaging device has also been increasing, it has been required to enhance the resolution and performance of the imaging device.
In JP-A-2002-221659, JP-A-2004-302057, JP-A-2004-341013, JP-A-2005-24581, JP-A-2005-24889, Japanese Patent No. 3424030 and JP-A-2007-17984, an imaging lens formed of three or four lenses is disclosed. As described in these patent documents, the known imaging lens includes four-lens configurations having an arrangement oft in order from the object side, positive, negative, positive, and positive power or having an arrangement of positive, negative, positive, and negative power. In the case of the imaging lens having the four-lens configuration, a lens closest to the imaging side mostly has a convex surface on the object side on the paraxial axis (in the vicinity of the optical axis). On the other hand, in Examples 5 and 9 of JP-A-2007-17984, there is disclosed a configuration in which a lens system has arrangement of positive, negative, positive, negative power and a lens surface closest to the imaging side is concave directed to the object side in the vicinity of the optical axis thereof.
As described above, recently, the imaging device has been reduced in size and has been increased in the number of pixels included therein. Especially, with respect to the imaging lens for a mobile camera module, there have been mainly required cost performance and compactness. However, recently, in the mobile camera module as well, the number of pixels included in the imaging device has been increasing and thus the need for the enhanced performance of the mobile camera module has been increasing. Thus, there has been desired the development of various lenses which are improved comprehensively in the cost, performance, and compactness thereof. For example, there has been desired the development of a low-cost and high-performance imaging lens which not only can secure compactness capable of use in a module camera for a cellular phone but also can provide a high performance capable of use in a digital camera. However, in these lenses described in patent documents mentioned above, for example, the image forming performance and compactness do not go together sufficiently. In addition, in JP-A-2007-17984, there is disclosed an imaging lens having various types of four-lens configurations, but it is hard to say that sufficiently optimized conditions are applied to the respective exemplary configurations.